The objective of the proposed research is to provide correlative morphological and biochemical information about the role of serotonin as a neurotransmitter. As a model system for this study, the autonomic nuclei of the spinal cord will be used. The synaptic patterns of these centers will be analyzed by quantitative electron microscopy, both in the adult and in the developing spinal cord. The synaptic profile thus derived will be compared with that in animals who have been treated with compounds known to cause destruction of the noradrenergic terminal (6- hydroxydopamine) or selective depletion or enhancement of terminal stores of monoamines (reserpine and monoamine oxidase inhibitors). An additional approach to the specification of the nature of the aminergic synapses on these neurons, electron microscopic autoradiography will be employed following pulse labeling with isotopic norepinephrine or serotonin. These experiments will be done on both the adult cord and the developing cord, choosing in the latter, the critical periods of development as regards amine formation. By these methods, it is expected that information will be derived as to the mode of termination of the two amine-containing pathways as well as to the spatial and temporal course of development of the innervation of the spinal cord autonomic centers.